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Modification of surface mechanical properties of IBAD-Cu Ni coatings
Thin Solid Films, !81 (! 989) 417~422
417
M O D I F I C A T I O N OF S U R F A C E M E C H A N I C A L P R O P E R T I E S O F I B A D - C u Ni C O A T I N G S J. VON STEBUT AND K. ANOUN
LSG2M/L.G.M., Ecole des Mines, F-54042 Nancy Cedex (France) J. P. RIVIERE AND R. J. GABORIAUD
Laboratoire de Mbtallurgie Physique, Facultb des Sciences, F-86022 Poitiers Cedex (France) (Received March 16, 1989)
Scratch testing of 1.4 lam CusoNiso films deposited on stainless steel by ion beam assisted deposition at room temperature shows the excellent adhesion obtained by this method. During long-term room temperature ageing spontaneous decohesion in and along the scratch tracks occurs together with a decrease in the pin/disc friction coefficient. First TEM results on cross-sectional cuts of the interface reveal some precipitates in the coating corresponding to f.c.c, solid solution CuNi.
1. INTRODUCTION
It is known that ageing may considerably modify microstructural and mechanical properties of surface layers produced by ion implantation, dynamic ion mixing (DIM) or ion beam assisted deposition (IBAD). For C ÷ implanted T a 6 A 1 4 V Vardiman 1 has shown that the size of the implantation-induced TiC precipitates strongly depends on the ageing temperature with optimum wear resistance occurring after 1 h at 400 °C. For C ÷ implanted austenite NislTi49 a beneficial effect was found by Popoola et al. 2 even after storage at room temperature; the number of cycles at breakthrough during pin/disc sliding wear increased from 5 to over 2000 after 6 months. Again TiC precipitate formation is supposed to explain this strongly enhanced wear resistance 3. In a recent TEM study C. Sarrazin et al. 4 concluded on the formation of an ordered, quadratic L1 o phase CusoNiso, IBAD film after ageing at 400 °C. In the present paper we discuss data for the same specimens after long-term ageing at room temperature resulting in increased brittleness and enhanced low-load wear resistance. This suggests that the same L 1o phase might already form under the present circumstances. 2. EXPERIMENTAL DETAILS 2.1. Specimen preparation and T E M investigations
The IBAD NiCu samples were prepared in an ultrahigh vacuum evaporation system with a base pressure of 10-6 Pa. Co-evaporation of copper and nickel was done by means of two computer-controlled electron gun sources. Ion beam 0040-6090/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
418
J. VON STEBUTet al.
enhancement was achieved in line by simultaneous operation of a 200 kV implanter yielding a beam of 60 keV Ar ÷ ions (current density: 1.5 laA c m - 2). TEM investigations were carried out with a Jeo1200 CX microscope. Vertical sections of the friction and wear specimens for TEM were prepared by means of mechanical CALOTEST thinnning, followed by standard ion etching S. 2.2. Friction and wear A conventional pin on disc test rig was run with a 5 mm diameter 100 C 6 steel ball (equivalent AIS152100) sliding in dry friction under normal load F N = 0.5 N on the implanted surface (CuNi on stainless steel). The sliding speed was 3 r.p.m. (,~ 30mm rain- 1). 2.3. Scratch testing Scratch testing was done under progressive loading by means of a CSEMRevetest, factory-equipped with an acoustic emission (AE) transducer and a light microscope (LM). This standard equipment was implemented by a special device based on a piezoelectric transducer allowing for friction force (FF) measurements. The tip radius of the scratch diamond was 200 I~m. AE and F F were monitored "on-line" for quick, roiatine failure detection. Detailed diagnosis of damage mechanisms was done "off-line" by means of LM, electron microprobe (EMP) and 3D profilometric surface mapping (cf. Refs. 6, 7). 3. RESULTS AND DISCUSSION
3.1. Contact mechanical properties Figure I shows scratch test data under progressively increasing normal load FN and illustrates the increase of coating adhesion by IBAD as compared with simple co-evaporation. Details on the corresponding FF and AE response can be found in Ref. 8. In the case of simple co-evaporation the film spalis offquickly after the onset Of the scratch track (FN ~< 1.5 N). It is worth mentioning that such brittle cohesive failure can be identified by means of the corresponding 3D surface mapping (Fig. l(b)) from steps in the surface profile allowing for a convenient "post-mortem" determination of the film thickness (about 1.4 lam). As to the IBAD film, it adheres everywhere, in spite of conformal brittle cracking and considerable pile-up along the side rims* beyond FN = 5N, corresponding to an overall track depth of 21am, slightly higher than the coating thickness. For such a film tested shortly after deposition a scotch tape pull-off test would not produce any decohesion in these piled up regions. Therefore it was surprising to discover that 18 months later, after storage in a drawer, spontaneous decohesion in these very regions had taken place. This situation is illustrated in Fig. 2, which shows the optical micrograph of a scratch track before ageing altogether similar to that in Fig. l(a). The spontaneous decohesions appear clearly on the EMP views of selected portions of this track (Fig. 2(b)).
* For a givenvalueof FNsuch pile-upcan be considerablyattenuated by choosinga harder substrate material.
SURFACE MECHANICAL PROPERTIES
6
419
p,m
Fig. 1. Scratch testing induced surface damage of 1.41am CusoNi~ o films deposited on stainless steel-influence of the deposition technique: (a) and (a*) optical micrograph; (b) and (b*) 3D surface mapping (inverted plots: z ~ - z); (a) and (b) co-evaporated specimen; (a*) and (b*) IBAD specimen without room temperature ageing.
S.E.M.O
FeK~ I
Fig. 2. Influence of ageing on the scratch testing induced surface damage of 1.4~tm Cus0Ni~o films deposited on stainless steel: (a) optical micrograph of the initial portion of a scratch track on the asdeposited IBAD film: (b) electron microprobe images of selected zones of (a) after 18 months ageing at room temperature. The Fe Kct X-ray analysis clearly reveals decohesion.
420
J. VON STEBUTet al.
Frequently one defines a critical load for brittle spalling or chipping Lc. In the present situation ageing would lead to a considerable decrease in L c, which is commonly taken as a qualitative measure of a system's behaviour with respect to friction and wear. To test such an assumption we have done low-load pin-on-disc friction testing. Figures 3(a) and 3(b) show that the initial friction coefficient of the as-deposited IBAD-film (lao = 0.35) is about three times that of the same film after 18 months' ageing (Ito = 0. I). In addition, during the first 20 revolutions la continues to rise for the as-deposited film (0.35 ~ 0.45) whereas it remains stationary for the aged film (0.14) after a run-in period of only two revolutions. Thus the low-load friction behaviour of the aged coating has become more favourable in spite of decreasing critical load due to ageing-induced spontaneous decohesion. The practical conclusion of these experimental results is that the structural and surface mechanical stability of I B A D coatings deposited at or below room temperature cannot be taken for granted. An analogous conclusion should probably be drawn for many other off-thermal-equilibrium surface treatments (e.g. normal ion implantation). Artificial accelerated ageing sufficiently above the projected future service temperature should be a solution to this problem.
3.2. Surface chemical and microstructural properties The pin/disc experiments of Fig. 3 were run under exactly identical contactmechanical conditions before and after ageing. Therefore we can assume that the ploughing component of friction was identical in both cases. The difference in the corresponding friction coefficients should thus be entirely adhesive in nature 8. This in turn suggests an ageing-induced 2 surface chemical and/or structural modification 9 of the CusoNiso I B A D films. Effects like surface oxidation during ageing can probably be excluded, otherwise the friction behaviour should be similar after 20 turns when a wear track of 0.3 lam in depth is measured by means of surface mapping 7. In addition, with a decrease in friction due to ageing, Lc would be expected to increase 1°'~ 1. This was not the case. Even after ageing the instantaneous results in scratch testing as evaluated by AE and FF as well as by LM and SEM inspection were the same as before. Therefore the ageing-induced delayed spalling might well be due to stressassisted microstructural transformations. As a starting point for the investigations of this complicated situation T E M was done on cross-sectional thin films of the coating/substrate interface region (Fig. 4). On the bright field micrograph (Fig. 4(a)) two distinct layers can be distinguished above a stainless steel substrate*. The coating diffraction pattern is given in Fig. 4(b). It corresponds to solid solution f.c.c. CuNi; the very faint additional ring could be interpreted as a (221) Llo superlattice reflection as in ref. 3 but this would certainly need confirmation. For the time being, evidence for an ordered L 1o structure likely to explain the observed ageing induced spalling and wear behaviour appears insufficient.
* The small coating thickness of approximately 300 nm is the residue left over after ion etching of the thin film TEM specimen.
SURFACE MECHANICAL PROPERTIES
42 |
.1 F r'!i......
i
I
(a)
I •
~
10
1S
~
Z5
J
~
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~
54
1-
(b)
l!
Z
,I
6
14 t6 15 ze ~l)l~ LZNOTn (nw.) ---~ Fig. 3. Pin-on-disc friction testing of CusoNi~o IBAD films--influence of ageing on the friction coefficient p as a function of sliding length: (a) as-deposited 1BAD film without ageing; (b) IBAD film after 18 months ageing at 300 K. 1o o
8
18
tZ
n~
lzZz
'
{ 200
/, /
~[(
zzl ) ?
(a) (b) Fig. 4. T E M on vertical cross-sectional cuts of CusoNlso IBAD films after 18 months ageing at room temperature: (a) bright filed micrograph across the substrate/coating interface; (b) diffraction pattern (DP) corresponding to the coating.
422
J. VON STEBUTet al.
4. CONCLUSION The contact mechanical results after long-term room temperature ageing prove that, for IBAD film off-thermal-equilibrium surface treatment, surface mechanical stability cannot be taken for granted, unless it is obtained by means of adequate tempering. With better quality cross-sectional TEM images forthcoming it should be possible to conclude whether the ageing effect observed is related to structural transformation in the IBAD coatings. ACKNOWLEDGMENTS
The authors wish to thank C. Fayoux and C. Boisseaux for assistance during IBAD, R. Rezakhanlou for his participation in scratch testing, J. P. Haeussler for E M P measurements and M. Foos for last minute debugging of D P results. REFERENCES 1 R . G . Vardiman, Mater. Res. Soc. Symp. Proc., 27 (1984) 699. 2 O. Popoola, P. Moine and J. P. Villain, Nucl. Instr. Meth., B26 (1987) 532. 3 O. Popoola, Ion implantation induced surface amorphization and precipitation in NisoTis0 alloys and their influence on tribological properties, Doctoral thesis, Univ. of Poitiers, June 1988. 4 C. Sarrazin, R. J. Gaboriaud and J. P. Riviere, Phys. Status. Solidi A, 107 (1988) 867. 5 K. Anoun, H. Michel and M. Gantois, Proc. 12th Worm Congress on Surface Finishing, lnterfinish 88, Paris 1988, p. 1063. 6 J. von Stebut, in E. Broszeit, W. D. Miinz, H. Oechsner K-T. Rie and D. K. Wolf (eds.), Plasma Surface Engineering, D.G.M. lnformations GES., Oberursel, F.R.G., 1989, po. 1215-1226. 7 J. von Stebut, J. P. Riviere, J. Delafond, C. Sarrazin and S. Michaux, Proc. Int. Conf. Surface modifications of metals by ion beams, Riva del Garda, Italy, 1988, Mater. Sci. Eng. A, 115 (1989) 267. 8 J. von Stebut, R. Rezakhanlou, K. Anoun, H. Michel and M. Gantois, Thin Solid Films, 181 (1989) 555. 9 I.L. Singer, Mater. Res. Soc. Proc., 27 (1984) 585 10 J. Valli, J. Vac. Sci. Technol. A, 4 (1986) 3007. 11 S.J. Bull, D. S. Rickerby, A. Matthews, A. Leyland, A Pace and J. Valli, Surf Coat. Technol., 36 (1988) 503 517.
417
M O D I F I C A T I O N OF S U R F A C E M E C H A N I C A L P R O P E R T I E S O F I B A D - C u Ni C O A T I N G S J. VON STEBUT AND K. ANOUN
LSG2M/L.G.M., Ecole des Mines, F-54042 Nancy Cedex (France) J. P. RIVIERE AND R. J. GABORIAUD
Laboratoire de Mbtallurgie Physique, Facultb des Sciences, F-86022 Poitiers Cedex (France) (Received March 16, 1989)
Scratch testing of 1.4 lam CusoNiso films deposited on stainless steel by ion beam assisted deposition at room temperature shows the excellent adhesion obtained by this method. During long-term room temperature ageing spontaneous decohesion in and along the scratch tracks occurs together with a decrease in the pin/disc friction coefficient. First TEM results on cross-sectional cuts of the interface reveal some precipitates in the coating corresponding to f.c.c, solid solution CuNi.
1. INTRODUCTION
It is known that ageing may considerably modify microstructural and mechanical properties of surface layers produced by ion implantation, dynamic ion mixing (DIM) or ion beam assisted deposition (IBAD). For C ÷ implanted T a 6 A 1 4 V Vardiman 1 has shown that the size of the implantation-induced TiC precipitates strongly depends on the ageing temperature with optimum wear resistance occurring after 1 h at 400 °C. For C ÷ implanted austenite NislTi49 a beneficial effect was found by Popoola et al. 2 even after storage at room temperature; the number of cycles at breakthrough during pin/disc sliding wear increased from 5 to over 2000 after 6 months. Again TiC precipitate formation is supposed to explain this strongly enhanced wear resistance 3. In a recent TEM study C. Sarrazin et al. 4 concluded on the formation of an ordered, quadratic L1 o phase CusoNiso, IBAD film after ageing at 400 °C. In the present paper we discuss data for the same specimens after long-term ageing at room temperature resulting in increased brittleness and enhanced low-load wear resistance. This suggests that the same L 1o phase might already form under the present circumstances. 2. EXPERIMENTAL DETAILS 2.1. Specimen preparation and T E M investigations
The IBAD NiCu samples were prepared in an ultrahigh vacuum evaporation system with a base pressure of 10-6 Pa. Co-evaporation of copper and nickel was done by means of two computer-controlled electron gun sources. Ion beam 0040-6090/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
418
J. VON STEBUTet al.
enhancement was achieved in line by simultaneous operation of a 200 kV implanter yielding a beam of 60 keV Ar ÷ ions (current density: 1.5 laA c m - 2). TEM investigations were carried out with a Jeo1200 CX microscope. Vertical sections of the friction and wear specimens for TEM were prepared by means of mechanical CALOTEST thinnning, followed by standard ion etching S. 2.2. Friction and wear A conventional pin on disc test rig was run with a 5 mm diameter 100 C 6 steel ball (equivalent AIS152100) sliding in dry friction under normal load F N = 0.5 N on the implanted surface (CuNi on stainless steel). The sliding speed was 3 r.p.m. (,~ 30mm rain- 1). 2.3. Scratch testing Scratch testing was done under progressive loading by means of a CSEMRevetest, factory-equipped with an acoustic emission (AE) transducer and a light microscope (LM). This standard equipment was implemented by a special device based on a piezoelectric transducer allowing for friction force (FF) measurements. The tip radius of the scratch diamond was 200 I~m. AE and F F were monitored "on-line" for quick, roiatine failure detection. Detailed diagnosis of damage mechanisms was done "off-line" by means of LM, electron microprobe (EMP) and 3D profilometric surface mapping (cf. Refs. 6, 7). 3. RESULTS AND DISCUSSION
3.1. Contact mechanical properties Figure I shows scratch test data under progressively increasing normal load FN and illustrates the increase of coating adhesion by IBAD as compared with simple co-evaporation. Details on the corresponding FF and AE response can be found in Ref. 8. In the case of simple co-evaporation the film spalis offquickly after the onset Of the scratch track (FN ~< 1.5 N). It is worth mentioning that such brittle cohesive failure can be identified by means of the corresponding 3D surface mapping (Fig. l(b)) from steps in the surface profile allowing for a convenient "post-mortem" determination of the film thickness (about 1.4 lam). As to the IBAD film, it adheres everywhere, in spite of conformal brittle cracking and considerable pile-up along the side rims* beyond FN = 5N, corresponding to an overall track depth of 21am, slightly higher than the coating thickness. For such a film tested shortly after deposition a scotch tape pull-off test would not produce any decohesion in these piled up regions. Therefore it was surprising to discover that 18 months later, after storage in a drawer, spontaneous decohesion in these very regions had taken place. This situation is illustrated in Fig. 2, which shows the optical micrograph of a scratch track before ageing altogether similar to that in Fig. l(a). The spontaneous decohesions appear clearly on the EMP views of selected portions of this track (Fig. 2(b)).
* For a givenvalueof FNsuch pile-upcan be considerablyattenuated by choosinga harder substrate material.
SURFACE MECHANICAL PROPERTIES
6
419
p,m
Fig. 1. Scratch testing induced surface damage of 1.41am CusoNi~ o films deposited on stainless steel-influence of the deposition technique: (a) and (a*) optical micrograph; (b) and (b*) 3D surface mapping (inverted plots: z ~ - z); (a) and (b) co-evaporated specimen; (a*) and (b*) IBAD specimen without room temperature ageing.
S.E.M.O
FeK~ I
Fig. 2. Influence of ageing on the scratch testing induced surface damage of 1.4~tm Cus0Ni~o films deposited on stainless steel: (a) optical micrograph of the initial portion of a scratch track on the asdeposited IBAD film: (b) electron microprobe images of selected zones of (a) after 18 months ageing at room temperature. The Fe Kct X-ray analysis clearly reveals decohesion.
420
J. VON STEBUTet al.
Frequently one defines a critical load for brittle spalling or chipping Lc. In the present situation ageing would lead to a considerable decrease in L c, which is commonly taken as a qualitative measure of a system's behaviour with respect to friction and wear. To test such an assumption we have done low-load pin-on-disc friction testing. Figures 3(a) and 3(b) show that the initial friction coefficient of the as-deposited IBAD-film (lao = 0.35) is about three times that of the same film after 18 months' ageing (Ito = 0. I). In addition, during the first 20 revolutions la continues to rise for the as-deposited film (0.35 ~ 0.45) whereas it remains stationary for the aged film (0.14) after a run-in period of only two revolutions. Thus the low-load friction behaviour of the aged coating has become more favourable in spite of decreasing critical load due to ageing-induced spontaneous decohesion. The practical conclusion of these experimental results is that the structural and surface mechanical stability of I B A D coatings deposited at or below room temperature cannot be taken for granted. An analogous conclusion should probably be drawn for many other off-thermal-equilibrium surface treatments (e.g. normal ion implantation). Artificial accelerated ageing sufficiently above the projected future service temperature should be a solution to this problem.
3.2. Surface chemical and microstructural properties The pin/disc experiments of Fig. 3 were run under exactly identical contactmechanical conditions before and after ageing. Therefore we can assume that the ploughing component of friction was identical in both cases. The difference in the corresponding friction coefficients should thus be entirely adhesive in nature 8. This in turn suggests an ageing-induced 2 surface chemical and/or structural modification 9 of the CusoNiso I B A D films. Effects like surface oxidation during ageing can probably be excluded, otherwise the friction behaviour should be similar after 20 turns when a wear track of 0.3 lam in depth is measured by means of surface mapping 7. In addition, with a decrease in friction due to ageing, Lc would be expected to increase 1°'~ 1. This was not the case. Even after ageing the instantaneous results in scratch testing as evaluated by AE and FF as well as by LM and SEM inspection were the same as before. Therefore the ageing-induced delayed spalling might well be due to stressassisted microstructural transformations. As a starting point for the investigations of this complicated situation T E M was done on cross-sectional thin films of the coating/substrate interface region (Fig. 4). On the bright field micrograph (Fig. 4(a)) two distinct layers can be distinguished above a stainless steel substrate*. The coating diffraction pattern is given in Fig. 4(b). It corresponds to solid solution f.c.c. CuNi; the very faint additional ring could be interpreted as a (221) Llo superlattice reflection as in ref. 3 but this would certainly need confirmation. For the time being, evidence for an ordered L 1o structure likely to explain the observed ageing induced spalling and wear behaviour appears insufficient.
* The small coating thickness of approximately 300 nm is the residue left over after ion etching of the thin film TEM specimen.
SURFACE MECHANICAL PROPERTIES
42 |
.1 F r'!i......
i
I
(a)
I •
~
10
1S
~
Z5
J
~
G
~
54
1-
(b)
l!
Z
,I
6
14 t6 15 ze ~l)l~ LZNOTn (nw.) ---~ Fig. 3. Pin-on-disc friction testing of CusoNi~o IBAD films--influence of ageing on the friction coefficient p as a function of sliding length: (a) as-deposited 1BAD film without ageing; (b) IBAD film after 18 months ageing at 300 K. 1o o
8
18
tZ
n~
lzZz
'
{ 200
/, /
~[(
zzl ) ?
(a) (b) Fig. 4. T E M on vertical cross-sectional cuts of CusoNlso IBAD films after 18 months ageing at room temperature: (a) bright filed micrograph across the substrate/coating interface; (b) diffraction pattern (DP) corresponding to the coating.
422
J. VON STEBUTet al.
4. CONCLUSION The contact mechanical results after long-term room temperature ageing prove that, for IBAD film off-thermal-equilibrium surface treatment, surface mechanical stability cannot be taken for granted, unless it is obtained by means of adequate tempering. With better quality cross-sectional TEM images forthcoming it should be possible to conclude whether the ageing effect observed is related to structural transformation in the IBAD coatings. ACKNOWLEDGMENTS
The authors wish to thank C. Fayoux and C. Boisseaux for assistance during IBAD, R. Rezakhanlou for his participation in scratch testing, J. P. Haeussler for E M P measurements and M. Foos for last minute debugging of D P results. REFERENCES 1 R . G . Vardiman, Mater. Res. Soc. Symp. Proc., 27 (1984) 699. 2 O. Popoola, P. Moine and J. P. Villain, Nucl. Instr. Meth., B26 (1987) 532. 3 O. Popoola, Ion implantation induced surface amorphization and precipitation in NisoTis0 alloys and their influence on tribological properties, Doctoral thesis, Univ. of Poitiers, June 1988. 4 C. Sarrazin, R. J. Gaboriaud and J. P. Riviere, Phys. Status. Solidi A, 107 (1988) 867. 5 K. Anoun, H. Michel and M. Gantois, Proc. 12th Worm Congress on Surface Finishing, lnterfinish 88, Paris 1988, p. 1063. 6 J. von Stebut, in E. Broszeit, W. D. Miinz, H. Oechsner K-T. Rie and D. K. Wolf (eds.), Plasma Surface Engineering, D.G.M. lnformations GES., Oberursel, F.R.G., 1989, po. 1215-1226. 7 J. von Stebut, J. P. Riviere, J. Delafond, C. Sarrazin and S. Michaux, Proc. Int. Conf. Surface modifications of metals by ion beams, Riva del Garda, Italy, 1988, Mater. Sci. Eng. A, 115 (1989) 267. 8 J. von Stebut, R. Rezakhanlou, K. Anoun, H. Michel and M. Gantois, Thin Solid Films, 181 (1989) 555. 9 I.L. Singer, Mater. Res. Soc. Proc., 27 (1984) 585 10 J. Valli, J. Vac. Sci. Technol. A, 4 (1986) 3007. 11 S.J. Bull, D. S. Rickerby, A. Matthews, A. Leyland, A Pace and J. Valli, Surf Coat. Technol., 36 (1988) 503 517.